The present invention relates to in-line joining of threaded rods in building construction. The rods can be “all thread” rods that are simply lengths of threaded rod, usually steel, bolts (e.g., anchor bolts that are embedded in concrete foundations), and combinations of the two. Holdown systems that link a series of threaded rods, from an anchor bolt through one or more threaded rods, are commonly used to secure a light frame building to the foundation walls or slab in high wind or earthquake prone regions. The anchor bolts pass from the concrete through holes in the mudsill, where they are typically tied to walls posts using holdown connectors. The threaded rods are connected with coupler nuts to the anchor bolts, and to each other in sequence with coupler nuts. The threaded rods pass through holes in the wall top plates up through the top story of the building, usually being secured at the bottom of each story and directly above the highest top plate. Because the uplift load reduces with elevation, the size of the threaded rods can also be reduced. For this reason, coupler nuts that allow one to connect threaded rods of different diameters are desirable. Coupler nuts that have two inner bore diameters are well-known, but these can only work in one orientation with respect to the connected rods. Furthermore, they can connect only two rods of two specific diameters that match the inner bore diameters of the coupler nut.